The present invention relates generally to a method and apparatus for oxygen concentration analysis.
Molecular sieve oxygen concentrators have become increasingly popular for the production of high purity oxygen (up to 95%) because of their simplicity, reduced energy consumption, and low operating costs; and in the future will replace conventional liquid oxygen converters as the primary source of breathable oxygen on-board aircraft. Also, they are used for patients requiring oxygen therapy. Molecular sieve oxygen concentrators are in use on-board military aircraft (U.S. Air Force B-1B, U.S. Navy AV-8B, and several NATO aircraft) for the production of oxygen to prevent hypoxia. These aircraft systems have become commonly known as On-Board Oxygen Generating Systems (OBOGS) or, more specifically, Molecular Sieve Oxygen Generating Systems (MSOGS). Nearly every future military aircraft will have an oxygen breathing system employing a molecular sieve oxygen concentrator.
These concentrators operate on the pressure swing adsorption technique and are capable of separating oxygen from an inlet stream of compressed air. On-board aircraft the compressed air is supplied as bleed air from the aircraft engine compressors. Using this technology, oxygen is separated from the inlet air by preferential adsorption of nitrogen within a molecular sieve. The nitrogen is vented overboard while the oxygen-rich product gas is breathed by the aircrew for the prevention of hypoxia. Use of these systems by the military has resulted in significant cost savings, enhanced aircraft versatility, and improved safety.
In their simplest form the molecular sieve oxygen concentrator is comprised of two cylindrical beds filled with a zeolite molecular sieve, several valves, and an orifice. The types of molecular sieves currently being employed are: 5AMG, MG3, 13X, and OXYSIV-5 (all manufactured by Union Carbide Corporation). The particle size of the molecular sieve pellets is generally 16.times.40 mesh. Typical aircraft oxygen concentrators contain approximately five to fifteen kilograms of molecular sieve depending on the amount of product flow required.
Because the oxygen concentration of the product gas is affected by the system operating conditions, such as, inlet pressure, product flow, ambient temperature, and the activity of the molecular sieve, molecular sieve oxygen concentrators generally require an oxygen concentration sensor. The sensor assures the oxygen concentrator is performing adequately for the specific application. Although several types of aircraft oxygen sensors have been proposed, such as, polarographic, fluidic, and zirconia, each has disadvantages when operating in an airborne environment.
One problem with present on-board generating systems is the lack of a reliable airborne oxygen sensor for monitoring the output of the oxygen concentrator. The sensor must have high reliability, low long-term drift, long operating life, and require little or no maintenance. Also, the sensor must be capable of performing in the aircraft environment where changes in temperature, pressure, vibration, and acceleration are always occurring. The sensor must be capable of powering up quickly and require little or no calibration except at the regular aircraft inspection intervals. Ideally, the sensor should be small in size, lightweight, inexpensive, and consume a small amount of electrical power.
The limitations and disadvantages of three types of oxygen sensors for aircraft use are listed below: (For more information on these three types, See Kocache, R. "The Measurement of Oxygen in Gas Mixtures," in "Survey of Oxygen Sensors for OBIGGS," by A. J. Meyer, Technical Operating Report #3, Appendix D, AF Aero-Propulsion Laboratory, Wright-Patterson AFB, OH (Contract F33615-84-C-2431)).
1. Polarographic oxygen sensor: This type of sensor functions by application of a voltage between a cathode and an anode which have been placed in an aqueous electrolyte such as, potassium chloride. The cathode is exposed to the sample gas which induces a redox reaction. The electrode reactions are: EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- (cathode) EQU 4Ag+4Cl.sup.- .fwdarw.4AgCl+4e.sup.- (anode)
Under the proper conditions the current in the cell varies linearly with the partial pressure of oxygen. The limitations and disadvantages of this type or aircraft oxygen sensor are:
a. Frequent calibration is required due to sensor drift.
b. The sensor has a limited life (generally 6 to 8 weeks).
c. The electrolyte may freeze.
2. Fluidic oxygen sensor: The fluidic oxygen sensor is dependent on the density and viscosity of the sample gas. In one configuration air is flowed into one side of a resistive bridge and the sample gas flows into the other side. The differential output pressure between the sample and reference gases is proportional to the oxygen concentration. The limitations and disadvantages of this type of aircraft oxygen sensor are:
a. Contaminants in the reference gas (bleed air), such as particulates, can clog the small passageways of the sensor These obstructions could significantly affect the sensor output.
b. The reliability of this type sensor has not been proven.
3. Zirconia oxygen sensor: This sensor is comprised of a sample chamber and reference chamber separated by a zirconium oxide disc. At high temperature the voltage between the electrodes, attached to each side of the disc, is related to the ratio of the partial pressure of oxygen at each electrode. This relationship may be expressed by the Nernst equation. ##EQU1## where,
R=universal gas constant
T=absolute temperature
F=Faraday constant
P"O.sub.2 =partial pressure of oxygen on the reference side
P'O.sub.2 =partial pressure of oxygen on the sample side
The limitation and disadvantages of this type of aircraft oxygen sensor are:
a. This sensor operates at a temperature of 1073 K. Operation at high temperature while exposed to high oxygen concentrations may pose a safety hazard.
b. Cycling this sensor on and off reduces sensor life.
c. This sensor requires a 10 to 20 minute warm-up time.